There is growing interest in first surface Enhanced Silver Mirrors, coatings on glass, Aluminium, Nickel, plastics, and possible other substrates. Silver has excellent reflectance in the visible and infrared spectral regions. Very thin Silver layers with adequate antireflective layers have unique characteristics of high reflectivity in the IR region and high transmittance (low reflectivity) in the visible region, which is desired for Heat Filters. Broad use of Silver coatings has been limited because of the poor mechanical, environmental and thermal durability of this material. The largest potential for Silver mirrors is in the Solar Energy industry. There is also a need for high reflectance coatings in the lighting industry, particularly for stage reflectors. Similarly, there is need for high performance IR Filters (Heat Filters) that are highly reflective from about 750 nm to 50 μm and above, and highly transparent in the visible spectrum from about 400 nm to about 700-750 nm. However Silver based coatings have serious durability and heat resistance problems resulting from outdoor climate in the case of Solar applications, and high operating temperature in the case of high power reflectors and heat filters. This novel, highly durable, and heat resistant Silver coating was designed to overcome some of the drawbacks in using Silver as a mirror or heat filter coating while retaining it's unique reflective characteristics.
Silver is a material with excellent optical properties. The use of Silver as an optical thin-film material is extensively described for example in the publication “Thin-film Optical Filters”, H. A. Macleod, Adam Hilger Ltd., Second Edition. Unfortunately, Silver has poor environmental and thermal compatibility, since it is, on the one hand, relatively soft and consequently can readily be mechanically damaged and, on the other hand, an impairment of optical properties occurs due to (chemical degradation) corrosion if the Silver is exposed without protection against the environment, a variety of chemicals, or elevated temperatures.
For this reason Silver layers are frequently interleaved in layered systems where the material selected for the remaining layers is determined by the desired optical properties, such as spectral properties, and also by the necessity of increasing the resistance of the Silver layer to environmental and mechanical degradation.
Oxides, Zinc Sulfide, Nitrides, Fluorides, or metals are frequently used in order to protect Silver in optical thin films. In particular, oxides are used due to their optical properties, their resistance to environmental factors, and also because of their hardness. Applying the oxide layer, however, can cause a degradation of the Silver. Much of the prior art has disclosed attempts to avoid this problem.
For example DR-OS-33 07 661 suggests first covering the silver layer with a further metal layer comprising aluminum, titanium, tantalum, chromium, manganese or zirconium, onto which further metal layers; and lastly an oxide layer is disposed, comprising indium oxide, tin oxide or a mixed oxide thereof. DE-OS-35 43 178 suggests a multilayer covering wherein the silver layer, in turn, is covered by a further metal layer comprising tantalum, tungsten, nickel or iron, which further metal layer, in turn, is covered by an oxide layer, wherein SnO, SiO.sub.2, Al.sub.2 O.sub.3, Ta.sub.2 O.sub.5 or ZrO.sub.2 are suggested as the oxide layers. Similarly U.S. Pat. No. 3,682,528 suggests covering the silver layer with a thin nickel layer, if any further layers are to be applied. According to an alternative embodiment, DE 33 27 256 suggests applying at least one hypostoichiometric oxide layer on the silver, comprising, for example, titanium oxide or titanium nitride or a mixture thereof. DE-A-33 29 504 further suggests covering the silver layer with a dielectric layer wherein the material composition in the region of the transition areas, changes gradually and continuously. Titanium oxide is mentioned; for example, as such a dielectric layer.
U.S. Pat. No. 5,510,173 describes substantially transparent copper and silver plus noble metal coatings. These coating's ability to withstand corrosive environments is improved by over-coating the metal layers with a double coating of dielectric. The first coating is made up of a dielectric based on indium and/or zinc, the second coating is made up of a dielectric based on indium and tin.
An environmentally stable silver containing mirror having very high reflection values over a large spectral range is disclosed in U.S. Pat. Nos. 6,275,332 and 6,128,126 which comprises a silver containing layer disposed on a substrate, which is covered by a zinc sulfide layer. To keep the sulfur from being set free during the application, or during the vaporization of the zinc sulfide, and attacking the silver, at least one barrier or intermediate layer is placed between the silver containing layer and the zinc sulfide layer.
U.S. Pat. No. 6,839,176 describes a process that starts with a standard substrate cleaning preparation with an abrasive or chemical cleaning method. The substrate is then transferred to a vacuum coating chamber. The substrate is then exposed to an Argon rich ion stream to further prepare the surface. Next a medium index material, or mixture of materials having a combined medium refractive index, is deposed on the surface with Argon ion bombardment. The Silver is then deposited until it is maximally reflective. A second deposition of the medium index material, or mixture of materials having a combined medium refractive index, is then done, also with the Argon ion bombardment. The second medium index material, or mixture, coating is then followed with a standard ion assisted optical film deposition to maximize reflection at the desired wavelength and angle of incidence.
The highly durable Silver coating for a mirror or IR thin-film optical filter of this invention exhibits better durability and, particularly, greater resistance to elevated temperatures than the prior art while still maintaining high reflectivity in IR and visible regions, or high reflectivity in IR and high transparency in visible spectrum as needed for particular application. For example the prior art's use of Zinc Sulfide is an undesired material for use in Silver based coatings as, in the presence of humidity, it may release chemicals corrosive to Silver such as H2S. Zinc Sulfide and some oxides such as ZnO, SnO are not very durable, consequently, coatings containing those material have low environmental durability.
Resistance of Silver based coating to elevated temperatures requires separate considerations. Some materials such as Zinc Sulfide decompose at temperatures exciding 400 C, particularly if moisture is present. Most oxides listed in the prior art may cause oxidation of Silver at elevated temperatures. Most typically, such oxidation is caused by the free oxygen trapped in the coating during reactive or semi-reactive deposition. At elevated temperatures such Oxygen diffuses through the layers and may easily reach and oxidize the surface of the Silver layer causing loss of reflectivity and de-lamination of layers. Typically oxides, such as TiO2, Ta2O5, Nb2O5, are deposited reactively or at least in presence of Oxygen (semi-reactively), leading to the process described above. Particularly damaging is if such oxides are deposited below the Silver layer (between the substrate and the Silver layer).